Speed limiting rotary coupling

ABSTRACT

A normally lubricated bearing, typically a ball bearing, serves to transmit torque from a drive shaft for the fan wheel of an alternator. The driving and driven bearing surfaces form the coupling parts respectively on the drive side and the delivery side of the coupling. A cup spring is useful to control the bearing friction that must be overcome at a limiting speed at which the drive shaft begins to turn faster than the driven device. Such couplings are useful for engine fans, pumps, or other similar auxilary devices in motor vehicles.

This invention concerns a coupling between a driving rotary member suchas a pulley on the fan belt of an internal combustion engine and adriven rotary member, such as the ventilating fan of an alternator, andparticularly a coupling that will slip at high speed, so as to limit thespeed of rotation of the driven member.

In motor vehicles, there are a large number of rotary devices driven atspeeds that vary greatly because of the variation of the engine speed,although for an optimal function of these devices a constant speed ofrotation would be desired. Examples of such devices are the electricgenerator, the engine fan, the water pump and the oil pumps. Couplingshave become known that operate by the centrifugal force of weightsrevolving around a shaft, arranged so as to weaken the contact betweenthe driving member and the driven member of the coupling after aparticular upper speed range has been reached and thus to prevent thespeed of the driven coupling member to increase further. Sucharrangements have been disclosed in German Patent No. 20 53 263 and inGerman patent application DE OS No. 28 22 638 of the assignee of thisapplication. In these known couplings, the power transmitting parts aresubjected to a degree of friction which causes a great deal of wear anda high heat load. The couplings are thereby unreliable and must receivecontinual attention and maintenance.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a speed limitingrotary coupling that produces little wear on the parts and generateslittle heat when it is driven at high speeds and therefore providesconsiderable slip. It is a further object to provide such a coupling ina form economic to manufacture and also in a form suitable for fittingon equipment already in service or already manufactured.

Briefly, the portion of the coupling that transmits force is a rotarybearing having one bearing surface connected with the drive side of thecoupling and another bearing surface connected with the driven side ofthe coupling, the bearing being lubricated and being subjected to apressure allowing the bearing to slip mainly at high driven speed. Aparticularly effective form of the coupling utilizes ball bearingsinterposed either between radial or axial bearing surfaces, and it isparticularly convenient to use two sets of ball bearings. The pressureis preferably applied by a suitable spring. Roller or needle bearingsare also usable, as well as sintered bearings, and so on. A particularlyuseful arrangement includes two single-row ball bearings of the angularcontact type with a spreading spring device between their outer rings,the bearings in this case being radial. In such an arrangement, it isparticularly useful to have one of the outer bearing rings axiallyshiftable. It is possible when two bearing units are used for thebearings to use different kinds of bearings, for example a needlebearing and a ball bearing, an arrangement that is particularly usefulfor a retrofit coupling.

It is also useful to provide a annular permanent magnet on the portionof the coupling on the drive side of the coupling, in order to modifythe coupling characterics by the generation of eddy currents.

The invention is further described by way of illustrative examples withreference to the annexed drawings, in which:

FIG. 1 is a sectional view of a motor vehicle alternator equipped with afirst embodiment of a coupling according to the invention, for drivingits ventialting fan;

FIGS. 2a and 2b show second and tird embodiments of coupling accordingto the invention having, like FIG. 1, a radial bearing;

FIG. 3 shows a fourth embodiment of a coupling according to theinvention, having an axial bearing;

FIG. 4 shows a fifth embodiment of a coupling according to theinvention, fitted to a drum-type blower for a generator;

FIG. 5 shows a sixth embodiment of a coupling according to the inventionconstituting a retrofit unit, and

FIG. 6 is a graph for explaining the characteristics of couplingsaccording to the invention.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

FIG. 1 shows, in cross-section, a three-phase a.c. generator having acasing that includes an end-bell 11 on the side of the drive and anend-bell 12 on the slipring side. A mounting or positioning arm 13,useful for swinging the generator about a pivot to set the belt tension,is provided integral with the drive-side end-bell 11. A statorlamination stack 15 is clamped between the end-bells 11 and 12 by meansof screws 14. Bearings 16 and 17 are seated in the respective end-bells11 and 12, and the generator shaft 18 is held in the bearings. Thestator 15 carries a stator winding 19. A rectifier, substantially all ofwhich is mounted on a cooling body 20, is connected to the outputs ofthe stator winding 19. The generator shaft 18 carries a rotor 21equipped with an exciter winding 22. The ends of the exciter winding 22are respectively connected to the sliprings 23 that are likewise mountedon the shaft 18. Carbon brushes held in the brushholder 24 are inoperative connection with the sliprings 23. The brushholder 24 iscombined physically in one unit with a voltage regulator 25.

Two further ball bearings respectively having inner rings 26 and 28 andouter rings 27 and 29 are fitted on the shaft 18 outside the casing11-12 on the drive side. The shaft 18 also carries on that side a beltpulley 31. Between the interior ring 32 of the bearing 16 and theinterior ring 28, between the interior ring 28 and the interior ring 26,and between the interior ring 26 and the pulley 31 there is interposedin each case a spacing ring 33. By means of a nut 34, the pulley 31, thespacing rings 33 and the interior bearing rings 26 and 28 are pressedagainst the interior ring 32 of the bearing 16.

The exterior bearing rings 27 and 29 are freely rotatable on the ball 35and on the exterior rings, a fan wheel 36 is seated. One of the twoexterior rings, preferably the exterior ring 29 on the side facing thebearing 16, is pressed into the hub of the fan wheel 36. The otherexterior ring 27 is then mounted in the hub of the fan wheel 36 in amanner which permits axial shifting.

The hub of the fan wheel 36 thus serves as the mounting for the exteriorrings 27 and 29. By means of a spring element 37 which bears against thefixed outer ring 29, the two external rings 27 and 29 are urged awayfrom each other. The force with which the exterior rings 27 and 29 areurged apart can be set by means, among others, of the thickness of thespacing ring 33a between the interior bearing rings 26 and 28. In theillustrated first embodiment, the spring element 37 is composed of twocup springs, of which one has its outer rim bearing against the exteriorbearing ring 29 and the other has its rim bearing against the exteriorbearing ring 27, while the inner edges of the two annular cup springslie adjacent to and against each other. An O-ring 38 is set between theshiftable outer ring 27 and the hub of the fan wheel 36.

The effect of the described coupling according to the invention is thatthe fan wheel 36 that pulls cooling air through the generator is notdriven directly, but through the frictional torque of the bearings26/35/27 and 28/35/29. As soon as the generator shaft begins to turn,the fan wheel 36 also gradually begins to turn. If the speed n_(w) ofthe shaft 18 then remains low and constant, the fan wheel 36 after ashort while reaches this rotation speed n_(w) of the shaft 18. Up to amaximum value n₁, that is essentially predetermined by the constants ofthe cup springs 37 and the width of the spacing ring 33a, the fan wheel36 then runs synchronously with the shaft 18. This speed value n₁ isdetermined by the essentially constant torque M_(K), that is necessaryto overcome the ball bearing friction. If now the speed n_(w) of theshaft 18 increases above the predetermined value n₁ already mentioned,then the speed n_(L) of the fan wheel 36 increases only insignificantlyfurther. In the graph given in FIG. 6, the torque M_(K) for overcomingthe bearing friction and the course of the torque M_(L) that isnecessary to drive the fan wheel are plotted against speed. At thealready mentioned speed value n₁, these two torques are equally large,thus M_(K) =M_(L) =M₁. In a speed range 0≦w≦n₁, the behavior of thecoupling can be described as synchronous drive in which n_(L) =n_(w) andM_(L) ≦M_(K), whereas the machine shaft speeds n_(w) <n₁ the behaviorcan be described as asynchronous drive, for which n_(L) <n_(w) and M_(L)>M_(K).

For the spring element 37, instead of cup springs, there can be used,for further examples, an undulated spring ring, a helical spring or someother such element. Even with such elements, it is possible by thechoice of the spring constant to help to set the speed up to which thefan wheel 36 should turn synchronously with the shaft 18. Since the ballbearings 26,27 and 28,29 do not turn while the shaft speed 18 isconstant up to a limiting value of shaft speed already mentioned, andabove this limiting value the speed of the fan wheel 36 is alwayssmaller than the speed of rotation of the shaft 18, the ball bearings26,27 and 28,29 can withstand whatever stress results from the increasedfriction losses of the ball bearings.

If a higher limiting value for synchronous speed of the fan wheel 36 isdesired, this can be obtained by the methods already described throughparallel disposition of several ball bearings between the shaft 18 andthe fan wheel 36, or by means of an eddy current device. Eddy currentdevices of the kind just referred to are illustrated in FIGS. 2a and 2b.In FIG. 2a, a permanent magnet ring 41 is so provided on the belt pulley31 that it stands opposite the disc shaped part of the fan wheel 36across a small air gap. At operating speed, the pulley 31 seeks in awell-known manner to pull the fan wheel 36 along with it. In FIG. 2b, amagnet ring 42 in the shape of a cylindrical sleeve is affixed to thepulley 31. The magnet ring 42 turns around the hub of the fan wheel 36with only a small air gap between them.

In the embodiments according to FIGS. 1, 2a and 2b, the drive-sidecoupling portion of the coupling are the shaft 18, 18', 18" and theinterior bearing rings 26, 26', 26" and 28, 28', 28", the torquetransmitting portion is made up of the interior rings 26, 26', 26" and28, 28', 28" the balls 35, 35', 35" and the exterior rings 27, 27', 27"and 29, 29', 29" and the output side coupling portion is made up of theouter bearing rings 27, 27', 27" and 29, 29', 29" and the hub of the fanwheel 36, 36', 36". The necessarily slanted races of the ball bearingsare here provided by the well-known angular ball bearing type ofconstruction.

In the foregoing paragraph the primed reference numerals refer to FIG.2a and the double-primed reference numerals refer to FIG. 2b, while theremainder (unprimed) refer to FIG. 1. Thus the bearings 26, 27 and 28,29 of FIG. 1 correspond to the bearings 26',27' and 28', 29' of FIG. 2aand to the bearings 26", 27" and 28", 29" of FIG. 2b.

In the embodiment according to FIG. 3, an axial bearing that operates onboth sides against a central driven member is utilized as the torquetransmitting portion. The function of the interior rings 26 and 28 ofFIG. 1 is here taken over by the rings 126 and 128, and likewise thefunctions of the exterior rings 27 and 29 by the rings 127 and 129. Inthis embodiment, the rings 127 and 129 are formed of one piece andconnected to the driven portion 126 of the coupling. The increase offriction is in principle obtained again by a cup spring operating as thespring element 137. The cup spring 137 in this third embodiment,however, is supported on a part that is fixed with respect to the shaft118 and presses the ring 128 against the axial bearing 126, 127,128,129.In this case also further measures, for example, an eddy current drive,can also be provided. In the embodiment shown in FIG. 4, a drum-typeblower is shown that is mounted on the slipring side of the generator.Components substantially the same as those of the first embodiment shownin FIG. 1 are designated with the same reference numerals. In this case,the bearings 226-229 utilize deep-grooved ball bearings. The drum-typeblower 236 runs in a blower casing 243 that is screwed onto the end-bell212 of the slipring side of the generator. The blower is of a known kindof construction and is therefore not further described here. Foradditional cooling of the hub of the blower 236 and therefore also ofthe ball bearings 226-229, additional blower blades 244 can be provided.

In the embodiments shown in FIGS. 1-4, only ball bearings hve beenshown, with rings that serve as bearing surfaces. The invention can bepracticed, however, also with the use of other rolling types ofbearings, for example roller or needle bearings. It is also possible touse, instead of rolling bearings, sliding bearings, for example sinteredbearings. In fact in each case illustrated, bearings of various kindsmay be used.

FIG. 5 shows a retrofit unit or kit utilizing a mass-productioncombination needle and ball bearing. Such a retrofit kit is readilymounted on the shaft 318 of equipment already manufactured, or alreadyin service, and is made up of merely three parts, the fan wheel 336, thebelt pulley 337 and the deep-grooved ball bearing 326, which also has abuilt-in needle bearing. The drive side generator bearing 332 and thespacer ring 333 are also indicated in FIG. 5.

Thus, it will be seen that although the invention has been describedwith reference to particular illustrative embodiments, furthervariations and modifications are possible within the inventive concept.

I claim:
 1. Electric generator for a motor vehicle, having a shaftdriven from the vehicle engine, having a fan for cooling the generatorand having, seated on said shaft a friction coupling for driving saidfan with a speed-limiting drive characteristic for said fan, which fancomprises:first and second bearings each having rolling bearingelements, a first bearing race ring (26,28; 126, 128; 226,228) supportedon said generator shaft and a second bearing race ring (27,29; 127,129;227,229) affixed to said fan; said first and second ring of each saidbearing being constituted so as to hold said bearing elements, as thelatter roll, at a constant spacing from said generator shaft, and springaction pressure-producing means (37) for exerting pressure parallel tothe axis of said shaft mounted for exerting said pressure as a relativeforce acting on said second rings (27,29; 127,129; 227,229) of both saidbearings.
 2. Generator according to claim 1, in which both said bearingsare radial bearings and said first rings thereof are radially interiorrings and said second rings thereof are radially exterior rings. 3.Generator according to claim 2, in which said said bearings are bothdeep-groove ball bearings.
 4. Generator according to claim 1, in whichsaid first and second bearings are oblique contact ball bearings, saidfirst rings thereof are radially interior rings and said second ringsthereof are radially exterior rings, and said spring action means (37)are constituted by at least compression-stressed member interposedbetween said second rings of said respective bearings.
 5. Generatoraccording to claim 3 or 4, in which a spacing ring (33a) is interposedbetween the respective said first rings (26,28) of said first and secondbearings.
 6. Generator according to claim 4, in which one (27) of saidsecond rings of said respective first and second bearings is axiallyshiftable with respect to said shaft (18,118) and to said fan (36,136).7. Generator according to claim 1 in which said first and secondbearings are axial bearings (126, 127; 128,129).
 8. Generator accordingto claim 7, in which said second rings (127,129) of said respectivefirst and second bearings are combined into an integral member and inwhich said spring action pressure-producing means (37) comprise at leastone compression-stressed member (137) provided between a collar at leastaxially fixed on said shaft and said first ring (128) of said secondbearing, the latter being mounted so as to be axially shiftable withrespect to said shaft.
 9. Generator according to claim 2, 4 or 8, inwhich said spring action pressure-producing means (37,137) isconstituted by at least one annular cup spring.
 10. Generator accordingto claim 9, in which said spring action pressure-producing means(37,137) is constituted by two cup spring parts put together in such away that the rim portions of said cup spring parts diverge from eachother.
 11. Generator according to claim 7, in which said bearingelements are balls.
 12. Generator according to claim 1, having alsopermanent magnet means (41,42) mounted in fixed position with respect tosaid shaft in such a way as to face a metallic portion of said fan (36),for inducing eddy currents in said metallic portion of said fan. 13.Generator according to claim 12, in which said permanent magnet means(42) is constituted by a cylindrical shell.
 14. Generator according toclaim 12, in which said permanent magnet means (41) is constituted by anannular body having an annular disk surface opposite said portion ofsaid fan (36).